Surgical Pathology Criteria

Leiomyoma of Deep Soft Tissue, Retroperitoneum, Mesentery and Omentum

Differential Diagnosis

In the deep soft tissue:

Leiomyoma (requires all below) Smooth Muscle Tumor of Uncertain Malignant Potential (used for any of below) Leiomyosarcoma (requires any one of below)
Cytologically bland   Cytologic pleomorphism or atypia
<1 mitotic figure / 50 HPF Bland but 1-4 mitotic figures / 50 HPF >4 mitotic figures / 50 HPF
No tumor cell necrosis Multiple recurrences but lacking other atypical features Coagulative tumor cell necrosis

Gastrointestinal Stromal Tumor (GIST)

Parasitic leiomyoma

Separation from myofibroblastic and fibrohistiocytic tumors is based on identification of smooth muscle differentiation:

 

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